1. Field of the Invention
The invention generally relates to plastic formulations. More particularly, the invention relates to medical grade plastic formulations suited for contact with human blood and its components.
2. Description of the Prior Art
Most whole blood collected is not stored and used for transfusion. Instead, the whole blood is separated into its therapeutically proven components, which are themselves stored in plastic containers. These therapeutic blood components of whole blood include red blood cells, platelets, and numerous other plasma-based fractions, such as albumen, plasma protein fraction, gamma globulen, and factor VIII.
Patient care is improved by providing only the therapeutic blood components of whole blood which are required to treat the specific disease. The demand for the therapeutic blood components of whole blood is thus ever-increasing. Likewise, the demand for safe and effective systems and methods for collecting, separating, and storing the therapeutic components of whole blood is growing.
Typically, whole blood and its components are collected and stored in containers made of medical grade plastic formulations, such as polyvinyl chloride (PVC) plastic formulations. These PVC formulations must, by necessity, include a plasticizer, because PVC is not itself flexible enough for use in these containers. Plasticizers are known to leach, or extract, into the blood components which are stored in PVC containers. While there are no known adverse physiological results which are linked to the leaching of plasticizers into the blood components, it is of course, desirable to minimize, as much as possible, the contact between blood components and any material not normally found in the human body.
PVC formulations also must, by necessity, include a heat stabilizer to prevent the PVC from undergoing heat degradation and color change during and after processing. Like plasticizers, heat stabilizers may also leach into the blood components. PVC plastic formulations including plasticizer and heat stabilization systems are described for example in U.S. Pat. No. 4,505,708.
Another important characteristic of blood component storage containers is gas-permeability. Gas permeability is essential so that the living cells of the blood component, such as red blood cells and platelets, can exchange oxygen and carbon dioxide. This allows for the extended viability of the living blood component and longer storage times. With regard to PVC plastic formulations, as the amount of plasticizer decreases, gas permeability generally decreases. Reduced gas permeability is not optimal for the storage of certain blood components, such as platelets.
As a result of the problems and shortcomings of PVC plastic formulations, several efforts have been made to develop plastic material suitable for storing blood components from non-PVC plastics. These materials include flexible polyesters and polyolefins. Surprisingly, many of the materials tested, while giving indications of being good plastic materials for the manufacturer of blood bags, have caused blood stored in the containers to exhibit an undesirably high plasma hemoglobin content. This indicates that the lysis rate of the red blood cells in these containers is high. Examples of blood bags made from plastic formulations other than PVC are disclosed in U.S. Pat. Nos. 4,112,982; 4,140,162; 4,301,800; 4,479,989; and 5,026,347. Several of these patents also disclose additional components to reduce the lysis of red blood cells, such as, citrate esters, and antihemolytic plasticizers incorporated into plastic inserts.
Sterilization of the blood component storage container is also of great importance. The containers should be preferably sterilizable by ethylene oxide, and/or radiation sterilization, such as gamma radiation sterilization. U.S. Pat. No. 4,479,989 discloses that, although plastic formulations including polypropylene are heat-sterilizable, they are undesirable since they may not be radiation-sterilized.
Furthermore, plastic formulations should remain flexible at low temperatures during storage. Plastic materials including polypropylene homopolyers, or copolymers, or blends thereof, are disclosed as brittle at low temperatures and inherently stiff. This is, of course, a disadvantage.
It would, therefore, be advantageous to provide a plastic formulation which can be formed by extrusion and/or blow molding to provide a medical fluid container which has a flexible, collapsible film body with good low temperature flexibility, sufficient temperature resistance to softening and/or melting, radiation and gas sterilizable, and sufficient gas-permeability to provide for the viability of the living blood components or other cellular matter therein. It would also be advantageous if the container had superior optical clarity and a combination of flexibility and strength which, imparts superior handling characteristics over a wide temperature range.
One aspect of the invention provides a flexible plastic formulation capable of being extruded and blow-molded, and radiation sterilized. The plastic formulation includes: a) from about 60 to about 94 percent by weight of a copolymer consisting essentially of ethylene and vinyl acetate units; b) from about 5 to about 25 percent by weight of a block copolymer, the block copolymer consisting essentially of: a central block of a copolymer molecule including generally equal proportions of ethylene and butylene units, and terminal blocks of polystyrene; and c) from about 1.0 to about 15 percent by weight of an ultra-low density polyethylene material.
Another aspect of the invention provides a flexible, collapsible container capable of being extruded and/or blow-molded, and radiation-sterilized, which is made from the plastic formulation of the invention.
A still further aspect of the invention is directed to a method of storing blood components, such as packed red blood cells or platelets, in a container made from the plastic material of the invention.